Two newly discovered black holes with masses equivalent to ten billion Suns could easily swallow five Solar Systems whole, report scientists in this week's issue of the journal Nature.

The black holes lurk in the bellies of two elliptical galaxies, NGC 3842, 320 million light years away in the constellation of Leo, and in NGC 4889, the brightest galaxy in the Coma Cluster, 336 million light years away. The previous record holder for most massive galaxy was 6.3 billion solar masses found in galaxy M87; the new contenders are at least 9.7 billion times more massive than our Sun, and possibly even larger. Furthermore, the event horizon, the point of no return, beyond which nothing can escape these black holes' immense gravity – not even light – is estimated to be five times the orbit of Pluto, and their gravitational influence would extend over a sphere 4,000 light years across.

Set in the heart of most galaxies, monster black holes are likely remnants of the brightest galaxies, or quasars, born from the death of a giant star that collapsed into a dense core, absorbing mass from its surrounds.

“In the early Universe, there were lots of quasars or active galactic nuclei, and some were expected to be powered by black holes as big as 10 billion solar masses or more,” explains Chung-Pei Ma of the University of California, Berkeley. “These two new supermassive black holes are similar in mass to young quasars, and may be the missing link between quasars and the supermassive black holes we see today.”

How could something so big remain hidden for so long? Ma and colleagues suggest that the once active monsters are now in "quiet retirement", their tumultuous gas-swallowing days long gone with stars forming out of the surviving gas and orbiting peacefully around the black hole.

Having studied galaxy mergers in computer simulations, Ma learned that the largest black holes are found in elliptical galaxies, which are thought to result from the merger of two spiral galaxies, but that the mergers of elliptical galaxies could spawn even greater supermassive black holes.

In search of real-life examples of these behemoths, Ma teamed up with observational astronomers using the Gemini and Keck observatories in Hawaii and at McDonald Observatory in Texas to study the starlight at the centres of several massive elliptical galaxies, and to analyse the velocity of the stars moving around their centres. They found the central masses to be in the range typical of quasars, but set within a volume of just a few hundred light years, leading to the conclusion that they were supermassive black holes, formed by the merging of galaxies.

“If all that mass were in stars, then we would see their light,” says Ma. “For an astronomer, finding these insatiable black holes is like finally encountering people nine feet tall, whose great height had only been inferred from fossilized bones. How did they grow so large? This rare find will help us understand whether these black holes had very tall parents or ate a lot of spinach.”